Rotating gantry CT systems have been the standard since the first systems were developed in the 1970s. While systems using this geometry are computationally efficient, they have several drawbacks, among them the mechanical complexity of the rotating components.
U.S. Pat. No. 7,593,502, issued to Katcha, et al. and assigned to General Electric Company on Sep. 22, 2009 for METHODS AND APPARATUS FOR FAST ROTATION SPEED SCANNERS is but one example of a rotating gantry CT system that includes a gantry having a stationary member coupled to a rotating member. The rotating member has an opened area proximate an axis about which the rotating member rotates. An x-ray source is provided on the rotating member. An x-ray detector may be disposed on the rotating member and configured to receive x-rays from the x-ray source.
In some ways, stationary or fixed gantry systems represent an improvement over rotating gantry systems, as the fixed gantry systems eliminate the need to rotate the massive x-ray sources and detector arrays and can make more efficient use of space by using a rectangular aperture. There are, however, concomitant disadvantages to fixed gantry systems. One of these is the variation in the flux normal to the detectors' surface due to variations in the distance to, and angle with, the source spot. In a typical fixed-gantry CT system, such as depicted in U.S. Pat. No. 7,280,631, issued to De Man, et al. and also assigned to General Electric Company on Oct. 9, 2007 for STATIONARY COMPUTED TOMOGRAPHY SYSTEM AND METHOD, the surface-normal flux can vary by as much as 3:1 across a one meter detector array. It is obvious that the signal-to-noise ratio of such a system could be improved by using a source that produced a more uniform flux normal to the detector array across its entire length. This can be achieved by the use of a collimator having a slit of non-uniform width.
Additional benefits can be realized in the slit design if a defocused source-spot is used. A larger spot produces a fan-beam more uniform across its central plane and allows the peak value to be varied through a greater range. This makes angular alignment across large distances less critical.
The use of a non-uniform slit in fixed gantry CT systems can therefore improve the signal-to-noise ratio and ease requirements for alignment of the x-ray source.